How to implement BADI for ME21N.....

Hi Experts,

I am trying to use the BAdi <b>ME_PROCESS_PO_CUST</b> for <b>ME21N</b> to extend its functionality, actually my requirement was to assign my own Purchase Order Number (<i>i am planning to assign different P.o number for different Plant</i>) whenever i create a PO....

Please help me on where and how to write the code for this requirement, i have created a implementation of the BAdi with the name <b>Z_ME_PROCESS_PO_CUST</b>, so i want to know in which method(<b>POST, CHECK,CLOSE</b> etc) of this BAdi i have to write the code for my requirement.

So all i need is to know where to assign the PO number so as i get my requirement.

Plz explain step by step as iam new to BAdi....

Thanks in Advance....

Regards

Karthik D